This application relates to a process for treating wastes from tanneries which convert hides to leather.
In the tanning of leather, the hides are first prepared by mechanical and chemical treatments to remove preservatives such as salt, hair, adhering flesh, and to remoisturize the hides. Such operations are generally known as "Beamhouse" operations and produce substantial quantities of waste water containing both dissolved and suspended organic matter from the hides and chemicals used in preparing the hides.
The thus prepared hides are then subjected to treatment with various tanning compounds to preserve the structure of the hide and prevent putrifaction. Trivalent chromium, typically hydrated basic chromium sulfate, is widely used. The trivalent chromium is often prepared at the tannery by reducing hexavalent dichromate with sulfuric acid and sugar. The cost of the chromium employed in tanning is a significant cost of operation. Substantial quantities are lost and carried away with the waste water resulting from the tanning and large quantities are also carried away with leather waste produced as scrap from trimming, shaving, buffing and the like. Such losses can amount to as much as two-thirds of the chromium employed in the process. Such loss is also an environmental hazard since all the ions of chromium are toxic, particularly the hexavalent ions. A typical tannery processing from 2000 to 3000 hides per day will lose about 2500 pounds of chromium (as Cr.sub.2 O.sub.3) per day, about 50% to 60% in the waste water and about 40% to 50% in the leather scrap.
The solid leather scrap containing chromium has heretofore been sold for conversion to fertilizer at prices approximating the cost of transportation for that purpose. The liquid wastes from the Beamhouse and tanning operations have generally been either passed directly to surface waters or combined in a separation tank where the suspended matter, usually aided by addition of precipitating or flotation agents, forms a wet sludge which is removed and disposed in landfill. Recent efforts to improve the handling of these wastes have concentrated on secondary and tertiary treatments of the liquid waste from the separation tanks and from dewatering of the wet sludge to reduce its volume and to retard the leaching of toxic materials such as chromium therefrom by weathering and anaerobic digestion.
For analysis, small samples of leather have been incinerated in closed laboratory furnaces at 600.degree. C., chromium extracted from the resulting ash with hazardous oxidizing acids, and the amount of chromium determined by titration. However, full scale disposal of sludge or leather scrap by incineration has not been considered practical because of cost, difficulty in burning organic matter including resulting odors, and because the operating temperatures of conventional incinerators tend to produce harmful quantities of toxic chromium which are emitted with the furnace exhaust gases. Typical conventional incinerators operate at temperatures of from 1400.degree. F. to 1700.degree. F. or higher.